[patterns] Support optional keys in Hash / objects

[ilyash-b]

Given the hash h, check that either key k is not present or, if it is present, it matches the pattern p.

Right now there is no straightforward way to check the above. Use case: id and params in https://www.jsonrpc.org/specification

[ilyash-b]

Idea: {"key": IfExists(pattern)}

[ilyash-b]

In dev